In order to produce high recording densities in hard disk drives (HDDs), recording bit length and recording track width of a recording medium may be reduced, in one approach. In order to read data from the recording medium having the recording bits with the reduced track width, without substantial amounts of error, a track width of a read head sensor (referred to as “read head track width”) and a stripe height (a depth from an air bearing surface of the read head) may also be reduced. The track width and the stripe height of the read head may be approximately 15 nm at a recording density of 2 terabytes per square inch (Tbpsi) and approximately 5 nm at a recording density of 5 Tbpsi.
The miniaturization of the read head sensor leads to a smaller volume of the magnetic body from which the read sensor is constructed. As a result, read noise is very undesirably large due to the increased magnetic instability caused by the anti-magnetic field of the magnetic body. In addition, miniaturization of the sensor reduces sensor sensitivity. Therefore, problems relating to degradation of the signal-to-noise ratio (SNR) and a significant increase in the error rate are also observable. The magnetic bodies forming the read sensor film may comprise an antiferromagnetic layer, a pinned layer, and a free layer. Improvements in the magnetic stability of these magnetic bodies would be very beneficial to magnetic head manufacturing and usage.
The sensor changes resistance by using a tunneling effect of electrons in an insulated barrier layer, which is referred to as tunneling magnetic resistance (TMR). The sensor film may comprise a seed layer, an antiferromagnetic (AFM) layer on the seed layer, a pinned layer formed on the AFM layer, an insulated barrier layer formed on the pinned layer, a free layer formed on the insulated barrier layer, and a cap layer formed on the free layer. The pinned layer has fixed magnetization by virtue of a strong exchange coupling field from the AFM layer, and there should be no fluctuation in magnetization during the signal reading process. The free layer should readily change the direction of magnetization according to a signal magnetic field from a recording medium comprising a special soft magnetic material.
During the process of reading a HDD, the magnetization of these magnetic layers may greatly fluctuate due to disturbances other than the signal magnetic field. Such great fluctuation in magnetization is undesirable, as it may be superimposed on the read signal as a read noise signal or cause errors in the reading process.
A magnetic moment of the pinned layer is pinned by an exchange coupling force from the AFM layer. Conventionally, a MnIr disordered alloy film may be used. The MnIr alloy film may have a face-centered-cubic (fcc) crystal structure with atoms arranged randomly in the structure. The miniaturization of the stripe height accompanying the miniaturization of the read head increases the demagnetizing field of the pinned layer, and the pinned layer becomes unstable, leading to fluctuations in magnetization sue to the disturbances. Therefore, sensor miniaturization and a stronger exchange coupling force are beneficial.
A large increase in the exchange coupling force between the AFM layer and the ferromagnetic layer of a L12 ordered alloy of Mn3Ir has been shown. K. Imakita et al., “Giant exchange anisotropy observed in Mn—Ir/Co—Fe bilayers containing ordered Mn3Ir phase,” Appl. Phys. Lett., 85, 3812 (2004). L12 ordered Mn3Ir is an alloy of Mn and Ir with an ordered placement of atoms that has a structure that places Mn in the center positions of the faces in a fcc lattice and places Ir in the corner positions. When a sputtering device is used to deposit film of MnIr at room temperature, a MnIr disordered alloy film is obtained. When growing an L12 ordered alloy of Mn3Ir, it has been reported that substrate heating, high gas pressure film deposition, and cold film deposition after MnIr deposition processes are required.
A film deposition chamber capable of high-temperature film deposition and a cooling chamber capable of cooling the substrate during the film deposition process, therefore, may be used to deposit the ordered film of Mn3Ir. The fabrication of an ordered alloy by hot film deposition has been attempted, and the fabrication of a L12 ordered alloy of Mn3Ir was confirmed. The results of X-ray diffraction measurements confirmed a degree of order representing the extent of the ordering ranges from 0.15 to 0.30, and a substantial increase in the value of the exchange coupling constant, Jk, that represents the strength of the exchange coupling force between the antiferromagnetic layer and the pinned layer from the conventional 0.6 erg/cm2 to 1.0 erg/cm2. Simultaneously, the blocking temperature, Tb, which is the temperature characteristic, substantially increased from 250° C. to 320° C.
Despite the increase in Jk over a conventional apparatus, however, the fabricated read head showed baseline fluctuation in the read signal waveform, which caused problems of read waveform instability. Fluctuation in the read signal waveform is called random telegraph noise (RTN), and is a noise produced randomly over time. RTN leads to read errors. Therefore, it would be beneficial to reduce the RTN which causes read errors.